1. Field of the Invention
The present invention relates to a capacitive occupant detection apparatus.
2. Description of the Related Art
A conventional capacitive occupant detection apparatus includes an occupant detection electronic control unit (ECU) and a capacitance sensor having a mat shape, and the capacitance sensor outputs electric current or voltage in accordance with a change in a weak electric field generated between a first electrode disposed in a seat and a vehicle body as described, for example, in JP-A-11-271463.
When the seat is empty, air is inserted between a pair of electrodes in the capacitance sensor. When a child restraint system (CRS) is disposed on the seat, the CRS is inserted between the pair of electrodes in the capacitance sensor. When an occupant sits on the seat, a body of the occupant is inserted between the pair of electrodes in the capacitance sensor. Air has a relative permittivity of about 1. A CRS has a relative permittivity of from about 2 to about 5, and the relative permittivity depends of a material of the CRS. A human body has a relative permittivity of about 50. As described above, air, a CRS, and a human body have different relative permittivities. Thus, a capacitance between the pair of electrodes in the capacitance sensor depends on an object inserted between the pair of electrodes.
The capacitance sensor outputs electric current or voltage in accordance with the change in the weak electric field generated between the pair of electrodes due to the difference in the capacitance. The occupant detection ECU determines whether the seat is empty, a CRS is disposed on the seat, or an adult sits on the seat based on the electric current or the voltage output from the capacitance sensor. An airbag ECU allows or prohibits an inflation of an airbag based on the determination result of the occupant detection ECU. When the occupant detection ECU determines that the seat is empty or a CRS is disposed on the seat, the airbag ECU prohibits the inflation of the airbag. When the occupant detection ECU determines that an adult sits on the seat, the airbag ECU allows the inflation of the airbag.
US 2005/0275202 A1 (corresponding to JP-A-2006-27591) discloses a capacitance sensor that detects whether a seat is wet so as to determine whether an occupant sits on the seat or the seat is empty more accurately. Water has a relative permittivity of about 80. Because the relative permittivity of water is greater than the relative permittivity of a human body, when the seat is wet, the occupant ECU is difficult to detect an occupant with accuracy. Thus, a sub electrode is provided in the capacitance sensor for detect a wetness. The capacitance sensor detects a change in an electric field between the sub electrode and a main electrode and outputs a detected result as electric current or voltage so that the occupant ECU can determine the wetness.
US 2006/0164254 A1 (corresponding to JP-A-2006-201129) discloses a capacitance sensor that includes a guard electrode for reducing a capacity. When a seat is empty, the capacitance sensor reduces electric current that flows between a pair of electrodes for an occupant determination. When an occupant sits on the seat, the capacitance sensor can detect electric current that flows between the pair of electrodes. When an occupant detection ECU applies a sine wave having a high frequency and a low voltage to the guard electrode and a vehicle ground, a capacitance is generated among the main electrode, a human body, and the vehicle ground, and the occupant detection ECU detects electric current in accordance with the capacitance.
In the capacitive occupant detection apparatus, a drift phenomenon may occur. In the drift phenomenon, properties of circuit components in the occupant detection ECU such as a signal source for generating a sine wave gradually change due to self-heating caused by operations of the circuit components and increase in ambient temperature. When amplitude of the sine wave generated from the signal source changes, the occupant detection ECU is difficult to detect an occupant with accuracy. JP-A-2000-153749 discloses an occupant detection system. Even when amplitude of a sine wave generated from a signal source changes due to the drift phenomenon, the occupant detection system can detect an occupant with accuracy by compensating the change in the amplitude.
In the occupant detection system, when a frequency of the sine wave applied to the capacitance sensor becomes abnormal, a capacitance value measured by the capacitance sensor becomes abnormal. However, the occupant detection system cannot detect abnormality of the frequency of the sine wave.